Compositor renders video zoomed in
You've just stumbled upon a unique feature of the compositor!

Yes, if your Scene>Dimensions>Resolution settings are not at 100% you won't see your full image.
The compositor doesn't automatically rescale images (or video) to fit the size used in the dimension's settings. When you render at 50% resolution, for example, it considers the composition to be half the number of pixels. The images in the image nodes get partially displayed because the picture size in pixels is not the same any more, but twice the size of the comp.

The workaround, if you want to work at a smaller resolutions, is to use a scale node with values that match your render settings. (But don't forget to turn it off a the end when you do the final render at full res!)